It is desirable to electronically control solar transmittance in windows to regulate solar heating and sunlight illumination. The sunlight spectrum is made up of an ultra-violet spectrum, a visible light spectrum, and a sunlight infrared spectrum. Generally it is desirable for windows to reject or filter the sunlight ultra-violet spectrum. Preferably, an electronic solution's visible-light functionality would include that provided by mechanical blinds or curtains; for example, selectable light states that include: see-through (i.e. transparent), privacy (opaque) or black-out, and reduced light transmittance. The sunlight infrared spectrum, the wavelength range from 0.78 to 2.5 microns, accounts for greater than 50% of solar energy, consequently its transmittance accounts for the majority of solar heating while contributing nothing to an inside environment's illumination. Indirectly thermal radiation (i.e. infrared typically 3 to 25 microns) from objects heated-up by sunlight in warm climates also contributes to heating of an internal environment.
Passive windows reflect thermal radiation by using a low-emissivity (i.e. low-e) coating on a glass pane in an insulating glass unit. Spectrally selective coatings are available for window panes that can reject the sunlight infrared spectrum. These coatings cannot be removed from a window when the function is not required unlike adjusting a mechanical blind. For example, it is highly desirable to reflect sunlight infrared when maintaining the temperature of an inside environment which requires cooling, but conversely, it is highly desirable to transmit sunlight infrared when maintaining the temperature of an inside environment which requires heating. It has been recognized for some time that it is desirable to have an active glass solution—a smart glass—that provides a variable range of visible light transmittance and solar transmittance.
The available smart glass solutions (also known as switchable windows) have limited functionality and inherent technological obstacles. For example, smart glass having a liquid crystal cell can be switched between a transparent state and a translucent state to provide privacy. If dichroic dye is in solution with the liquid crystal a light absorbing state can be implemented. The available solutions have liquid crystal dispersed discretely or semi continuously in a polymer matrix film and are referred to as polymer dispersed. Examples include Polymer Dispersed Liquid Crystal (PDLC), emulsion based processes often referred to as Nematic Curvilinear Aligned Phase (NCAP), and Polymer Induced Phase Separation (PIPS). A polymer dispersed liquid crystal film is laminated between glass panes to make a smart glass. In the prior art of liquid crystal, smart glass there is no adequate variable light transmittance solution, or no solution offering selective control of solar transmittance.
Suspended Particle Devices (SPD) is another prior art smart glass technology. It employs anisotropic (i.e. rod-like) dipole-particles in a suspension that align in the presence of an electrical field similar to liquid crystal molecules, and the dipole-particles do not move position or translate in a field as is the case with charged-particles in electrophoretic devices as described later. An SPD suspension is dispersed in a polymer matrix film that is subsequently laminated to glass panes. SPD smart glass provides a variable light transmittance function (dimming while maintaining transparency) but does not provide privacy (not opaque or translucent), and does not provide active control over infrared (i.e. SPD devices do not reflect infrared light). An inherent technological obstacle with SPD devices in the prior art is that their light absorptance is not neutral in the visible light range with the consequence that their dimming states appear somewhat blue and their fully darkened state appears dark blue.
There are a range of electrochromic smart glass technologies. These are not the simple light switches of other technologies; instead a reversible chemical reaction occurs when changing light states. Typically these solutions are deposited as an active layer on a glass pane, and the active layer must be provided in the sealed cavity of an insulated glass unit. Electrochromic smart glass IGUs provide some selectable control over visible and infrared light. The visible light function provided is variable light transmittance (i.e. dimming with transparency). A privacy function is not available (not opaque). Undesirably, light absorptance is not neutral for the visible light spectrum causing the variable light transmittance states to appear somewhat blue and the fully darkened state to appear dark blue.
Electrophoretic technology would seem to have the advantage of a black state but the prior art does not provide feasible solutions for smart glass. Conventional electrophoretic devices move charged particles in a suspending fluid in the direction of an electrical field. This is normally an orthogonal field from a front electrode face to a rear electrode face (i.e. vertical movement) and a transparent state is not possible. One proposal to create a transparent state in an electrophoretic device is to finely pattern one of its electrodes. This allows the charged particles to be moved laterally as well as vertically and to collect on patterned electrodes (corresponding to a subset of a display area) and having a pitch of about 200 to 300 microns. The area between patterned electrodes is then transparent and provides visual access. In some examples of display devices employing patterned electrodes just one substrate has electrodes and particles move laterally between neighbouring electrodes with one electrode group accounting for about 70% of the area. But while the fine patterning of electrodes is normal for display devices having a matrix of pixels, it is prohibitively expensive for a device intended for smart glass applications.
Other examples of electrophoretic devices that may have an inherent transparent light state capable of transmitting light and providing visual access include electrophoretic devices that use a dielectrophoretic effect to collect charged particles at a side wall of a capsule in one light state; or electrophoretic devices that use replicated microstructures (e.g., using one of the following processes: embossing, photolithography, extruding, or laser micromachining) to collect charged particles in one light state; or electrophoretic devices that use the dispersal (i.e. in a suspending fluid volume) of 10 nm to 50 nm scale, charged nanoparticles to transmit light and provide visual access in one light state; or hybrid electrophoretic devices (called electrokinetic by their inventors at Hewlett Packard) that use photolithographically created micro-pits to collect charged particles in one light state. The feasibility of these prior art electrophoretic technologies for smart glass applications is questionable due to the efficacy of their transparent light state or their complexity. For example, one proponent of replicated microstructures proposes making embossing molds on silicon. This would seem to limit such devices to small areas and discrete or batch manufacturing, and such tooling would not seem suited to the large-format, roll-to-roll manufacturing desirable for smart glass.
At ground level in hot climates the energy level in the three sunlight spectrums per square meter with the sun at its zenith is about 32 watts (3%) of UV, 445 watts (44%) of visible, and 527 watts (53%) of infrared, or about 1,004 watts total. In general, the prior art of smart glass does not describe what happens this solar energy when it is absorbed by light states, does not provide the means to manage and adapt the visible light spectrum and the sunlight infrared spectrum for a climate, and does not provide solutions that direct the energy from absorbed sunlight to substantially an outside or inside environment as required. Furthermore, the prior art of smart glass is largely silent with respect to managing the heat build-up within a device that absorbs incident solar energy.
In summary, there is a need for a solar control device that provides variable sunlight transmittance, variable solar-energy transmittance, colour-neutral visible light transmittance, the ability to direct the sunlight energy its light states absorbs to substantially an outside or inside environment as required, the ability to manage and adapt its sunlight transmission spectrum for a climate, and the ability to minimize and manage heat build-up within a device.